1. Field of the Invention
The invention relates generally to the field of optical acceleration and inclination sensing devices. More particularly, the invention relates to optical accelerometers and inclinometers used for, but not limited to, sensing seismic energy.
2. Background Art
Optical sensing devices for measuring parameters such as acceleration, motion and/or pressure are used for, among other purposes, detecting seismic energy from the Earth's subsurface. The seismic energy may be naturally occurring, or may be imparted into the Earth by a seismic energy source for the purpose of performing reflection seismic surveys. Detecting seismic energy may include measuring pressure, or changes in pressure with respect to time, in a body of water. A sensor used to measure such changes in pressure is known as a hydrophone. Detecting seismic energy also includes detecting motion on or near the Earth's surface. Motion may be detected using devices known as geophones or accelerometers. Geophone signals are related to velocity of motion. Accelerometers produce signals related to the time derivative of velocity of motion, which is acceleration. Inclinometers, which produce signals related to the relative orientation of a device with respect to the direction of Earth's gravitational pull, are sometimes used in association with other sensors to determine the gravitational orientation of any device associated with the inclinometer.
Sensors known in the art which respond to the foregoing physical parameters generate an optical signal in response to the detected physical parameter. The optical signal may be, for example, a change in reflected wavelength, a change in phase or an interference pattern in response to changes in the physical parameter.
Generally, optical sensors known in the art include a selected length of optical fiber affixed to a device that changes shape in response to changes in the physical parameter being detected. The change in shape of the device is transformed into a change in length of the optical fiber. Change in length of the optical fiber may be detected by one of a number of different optical measurement techniques. Such techniques include change in reflected wavelength of light as a result of a change in wavelength of a Bragg grating formed in the optical fiber, or optical coupling of a light beam transmitted through the optical fiber with a light beam transmitted through another optical fiber, known as a “reference fiber.” The reference fiber may be disposed such that its length remains essentially unchanged irrespective of the value of the physical parameter. Light beams from the fiber affixed to the device and from the reference fiber are coupled in an optical interferometer. An interference pattern or phase change in the light generated in the optical interferometer is related to the change in length of the fiber coupled to the device, and thus to the physical parameter being measured. Typically the output of the interferometer is coupled to a photodetector, which generates an electrical signal related to the light amplitude applied to the photodetector.
A fiber optic hydrophone is disclosed, for example, in U.S. Pat. No. 5,625,724 issued to Frederick et al. The hydrophone disclosed in the Frederick et al. '724 patent includes a reference fiber wrapped around a rigid inner cylinder. A solid layer of compliant material is applied over the reference fiber. The sensing arm of the interferometer is wound over the layer of material applied over the reference fiber. The outer material is sufficiently compliant to provide acoustic sensitivity comparable to that of air-backed hydrophones.
Another fiber optic hydrophone is disclosed in U.S. Pat. No. 6,549,488 issued to Maas et al. and assigned to the assignee of the present invention. A hydrophone made according to the Maas et al. '488 patent includes a compliant sensing mandrel coaxial with and adjacent to a rigid reference mandrel. A first optical fiber is wound around the compliant sensing mandrel. A second optical fiber is wound around the reference mandrel. The first and second optical fibers comprise different arms of an interferometer. Flexible sealing members, such as O-rings, seal the compliant sensing mandrel to the rigid reference mandrel. In one embodiment, one O-ring is disposed near each end of the sensing mandrel. A cylindrical support member is disposed inside the sensing mandrel. At least a portion of the support member is spaced from the sensing mandrel so as to provide a sealed cavity between the sensing mandrel and the support member. The sealed cavity is filled with air or similar compliant substance.
U.S. Pat. No. 5,369,485 issued to Hofler et al. discloses an optical accelerometer wherein an elastic disk and a predetermined mass are supported by a body for flexure of the disk due to acceleration, shock, vibration and displacement of the body in a direction axially of the disk. Such a disk, or a plurality of such disks, are wound with a pair of flat spirals of optical fiber, each spiral being fixedly attached to a corresponding disk side so that disk flexure lengthens a spiral on one disk side and shortens a spiral on another disk side. Such spirals on oppositely facing disk sides are connected as opposite legs of a fiber optical interferometer so that the interferometer provides an output corresponding to the amplitude of the flexure. A “push-pull” pair of the spirals may be disposed oppositely of a thermally conducting disk to minimize temperature differences between the push-pull spiral pair. An accelerometer according to the disclosure in the Hofler et al. patent is constructed with a centrally supported disk having the mass distributed around the disk periphery. Such construction is purported to be advantageous for isolation from mounting stress and for providing a plurality of coaxially mounted disks for increased sensitivity.
U.S. Pat. No. 6,650,418 issued to Tweedy et al. discloses a fiber optic sensor that includes a flexural disk having a pair of fiber optic coils mounted on opposite sides thereof and optically coupled together to form an interferometer that produces an output signal in response to acceleration of the flexural disk. The accelerometer includes a housing having first and second end plates with a sidewall extending between the end plates. The sidewall has an inwardly facing groove in which an outer edge portion of the flexural disk is mounted. A compressive damper is mounted in the housing and arranged to exert a compressive force on the flexural disk to control movement thereof in response to acceleration of the flexural disk along a sensing axis and thereby control the output signal.
U.S. Pat. No. 6,575,033 issued to Knudsen et al. discloses a highly sensitive accelerometer, which includes a mass within a housing suspended by opposing support members. The support members are alternately wound around a pair of fixed mandrels and the mass in a push-pull arrangement. At least a portion of one of the support members comprises optical fiber coils as the support members for use in interferometric sensing processes.
More recently, multiple-direction sensitive (“multicomponent”) motion sensors disposed on a cable in conjunction with substantially collocated hydrophones have been used on the bottom of a body of water for marine seismic surveying. Such cables are known in the art as “dual sensor OBCs.” See, for example, U.S. Pat. No. 6,314,371 issued to Monk, which discloses a method for processing of dual sensor OBC data that corrects for energy incidence angle, corrects for estimated reflectivity, and combines corrected seismic sensor traces using an optimal diversity scaling technique. In one embodiment, the disclosed method takes seismic traces from a geophone and a hydrophone, corrects the geophone trace for the incidence angle, determines diversity filters for optimally combining the geophone and hydrophone traces, applies the diversity filters, estimates a reflectivity coefficient for the ocean bottom (potentially for different angles of reflection), scales the geophone data according to the reflectivity, and re-applies the diversity filters to obtain a combined trace. The combined trace is expected to have various artifacts eliminated, including ghosting and reverberation, and is expected to have an optimally determined signal-to-noise ratio.
It is important that motion sensors in general, and in particular those sensors used in dual sensor OBCs, have good sensitivity, are relatively insensitive to noise, and have good rejection of cross-component signal (meaning that the motion sensors are substantially insensitive to motion along any direction other than the sensitive axis). Accordingly, there is a continuing need for motion and/or acceleration sensors that have improved sensitivity, reduced noise and reduced cross-component sensitivity. More recently, an improved optical acceleration sensor particularly suited for use with OBCs has been devised by Steven J. Maas and D. Richard Metzbower, as more fully described in U.S. patent application Ser. No. 11/095,860—filed on Mar. 31, 2005 and assigned to the assignee of the present invention. Such improved optical acceleration sensor includes a beam and at least one optical fiber affixed to one side of the beam such that deflection of the beam changes a length of the optical fiber. Means for sensing the change in length of the optical fiber is functionally coupled to the at least one fiber.
One common limitation to substantially all motion and acceleration sensors known in the art for use with OBCs and other submerged sensing systems is that they are typically disposed in a pressure resistant housing. The pressure resistant housing is adapted to exclude water under high pressure, such as caused by submersion of the sensor at great water depth (approximately 3000 meters or more) from entering the housing. An interior of such housings is generally maintained at surface atmospheric pressure (about 1 bar). As a practical matter, housings having the capability of excluding water under pressure such as at the foregoing submersion depths must be made from steel or similar high strength material, and must have relatively thick walled construction to avoid crushing under pressure or leakage. Such construction is expensive, and makes any sensor system such as an OBC used therewith heavy and difficult to deploy. Accordingly, there exists a need for improved optical motion sensing devices that can be immersed to great water depth, while avoiding the expense and difficulty of construction of pressure resistant housings for the sensors.